Neural stem cells residing in specialized niches in the adult mammalian brain may represent a large pool of cells that can be stimulated for brain repair. Adult neural stem cells continuously generate new neurons that functionally integrate into neural circuits. Defining the molecular events that underly stem cell self-renewal and differentiation are essential steps towards harnessing the endogenous stem cells and progenitors for brain repair. While recent progress has been made in identifying transcriptional pathways regulating these processes, post-transcriptional regulatory mechanisms are almost completely unknown. MicroRNAs are a novel class of small non-coding RNAs that largely act by repressing translation. We hypothesize that miRNAs are key mediators of stem cell biology and cell fate decisions and act in concert with transcriptional regulators to control adult neural stem cell self-renewal and differentiation. We propose here to 1) characterize the functional role of 3 miRNAs we have identified in the adult neural stem cell lineage. We will use retroviruses to ectopically over-express and knock down these miRNAs in cultured neural stem cells to examine their functional role in stem cell self-renewal and differentiation. We will determine the functional consequences for adult neurogenesis in vivo by retroviral over-expression and knock down of these miRNAs. 2) We will identify the physiological targets of these miRNAs, which will yield important insight into the networks regulated by miRNAs. 3) We will clone and characterize specific libraries of miRNAs for each cell stage in the lineage of adult neurogenesis. These studies will yield key insights into the function of miRNAs in stem cells for adult neurogenesis.